The present invention relates to a nebulizer, or an ultrasonic atomizer which converts water to mist. In particular, the present invention relates to a self-oscillation circuit for energizing a piezo-electric vibrator.
A prior self oscillation circuit for energizing a piezo-electric vibrator for a nebulizer is shown in Japanese utility model publication No. 12619/85 and/or U.S. Pat. No. 3,989,042. That circuit has been used for conveniently providing moisture in a room.
FIG. 4 shows a prior nebulizer circuit which is a grounded collector transistor type self oscillation circuit (Colpitts oscillation circuit). In the figure, the symbol Q1 is a transistor, R1 through R6 are resistors, C1 through C6 are capacitors, L1 through L3 are inductors, and TD is a piezo-electric vibrator for generating ultrasonic wave vibration. The vibrator TD is mounted at a bottom of a water tank which contains water to be atomized, and therefore, one of the electrodes of said vibrator is coupled with a collector of the transistor Q1, and said electrode contacts directly with water. The oscillation circuit of FIG. 4 is supplied with D.C. voltage which is obtained by rectifying A.C. voltage (for instance A.C. 48 volts) by using a fullwave rectifier DS. Exactly speaking, the supply voltage is pulsating voltage, but is not flat DC voltage, since the capacitance C1 is rather small and not enough for supplying flat DC voltage, for reducing the manufacturing cost of the circuit. The capacitance C1 is for instance 0.1 .mu.F, which operates to short-circuit between the collector of the transistor Q1 and the junction point of the inductor L1 and the resistor R3 for the oscillation frequency (for instance 1.65 MHz) of the vibrator TD.
The inductor L3 and the capacitor C4 coupled with a base circuit of the transistor Q1 operates as a noise filter, which is useful to prevent noise on a line between the junction point of the resistors R2 and R3, and the base circuit of the transistor Q1. That line might be long, when a variable resistor for adjusting amount of mist is coupled between said junction point and said base circuit, and might induce undesirable noise.
A small nebulizer which atomizes 400 cc/hour, or 500 cc/hour used in an ordinary house has a single nebulizer circuit of FIG. 4 for each nebulizer. There are no problems in that case.
On the other hand, a large nebulizer which atomizes more water has a plurality of oscillation circuits. If there is no interference among those oscillation circuits, the amount of water to be atomized by said large nebulizer is n times of that of a single oscillation circuit, where n is a number of oscillation circuits.
However, when a power transformer PT is used commonly for all the oscillation circuits as shown in FIG. 5, we have realized the following disadvantages.
FIG. 6 shows operational waveforms of the circuit of FIG. 5. FIG. 6A shows the waveform at the point T at one end of the vibrator TD. That waveform is the pulsation voltage having the period of 10 mS (100 Hz) which is twice of the power supply frequency (in case of fullwave rectification). A small drift D is observed in the waveform in FIG. 6A because of the saturation voltage between the collector and the emitter of the transistor Q1. The drift D is not important in the present invention.
FIG. 6B is the enlarged view of the portion X of FIG. 6A.
The high frequency vibration of 1.65 MHz which is applied to the vibrators TD is observed in FIG. 6A and 6B.
It should be noted in those figures, in particular in FIG. 6B, that the amplitude of the high frequency signal of 1.65 MHz fluctuates with about 20 kHz (period is 20 .mu.S). That fluctuation frequency of 20 kHz depends upon a value of circuit elements, and distributes between 5kHz and 100 kHz. That undesired ripple fluctuation decreases the amount of the mist, since the vibrator is not energized by the full voltage at the portion Z where the voltage applied to the vibrator TD is rather low. The amplitude of that ripple voltage depends upon the value of circuit elements, and is high enough to decrease the amount of the mist considerably.
The amplitude of the waveform of FIG. 6A and FIG. 6B is 100-150 volts when the power supply voltage is 48 volts (2.times.1.41.times.48). However, the amplitude of the fluctuation is not small as compared with that of the voltage, and the fluctuation decreases the amount of the mist.
That undesired ripple voltage is observed whether vibrators are mounted on separate water tanks, or they are mounted on a large common water tank.
In FIG. 5, the conductor GD which couples all the collectors of all the transistors commonly is provided by the conductive water in a tank. We have also observed that when the collectors of all the transistors are coupled commonly with a conductor line GD, the ripple vibration is strong. However, even if the conductor line GD is not used and a plurality of separate water tanks are used, the ripple is observed as far as a plurality of oscillation circuits are coupled with a common rectifier which is connected to a single transformer.
As mentioned above, a prior oscillation circuit has the disadvantage that when a plurality of oscillation circuits are coupled with a single common power transformer PT, since those oscillation circuits interfer with one another, an undesired ripple oscillation of 5 kHz-100 kHz is generated. Thus, the total amount of mist is less than n times of that of each of the oscillation circuits, even when n number of oscillation circuits are used. Further, the amount of decrease of mist depends upon circuit elements, and the accurate design of the amount of mist was impossible.